1. Field of the Invention
The present invention relates to an optical multiplexing/demultiplexing device for coupling an end face of a first optical waveguide which transmits a number N of signals of different wavelengths with the end faces of N further optical waveguides which each transmit one of said signals, which device comprises means for spectrally separating said signals, which spectral separating means comprises a number N of wavelength selective reflective interference filters and means for spatially separating the N reflected signals.
The invention may be utilized for the multiplexing and demultiplexing of signals for telecommuniaction purposes.
2. Description of the Related Art
Such a device is disclosed in British Patent Specification No. GB 2096 350 A published on the 13th of October 1982. This Specification describes a coupling device for coupling signal-transmission elements, specifically optical fibres, which device comprises means for selecting the transmitted signals as a function of their wavelengths. These means comprise a lens and a plurality of non-parallel interference filters arranged after one another. Coupling between a fibre which transmits a plurality of signals of different wavelengths and a plurality of fibres which each transmit one signal is achieved in that the interference filters are arranged on adjacent prism faces so as to define their spacing and their orientation. The prism is a cylindrical graded-index lens by means of which each of the beams of different wavelengths can be focused on the corresponding surfaces of the selection filters.
This known device has several drawbacks. The principal drawback is that the adjustment of such a device must be effected by a displacement of the optical fibres, which are elements having dimensions of the order of one hundred microns. This adjustment is extremely critical. Another disadvantage is that the lens is a graded-index cylindrical lens. It is well-known to those skilled in the art that this element provides only an approximate focusing, because sometimes the refractive index may deviate. This gives rise to insertion losses upon coupling. Moreover, this lens must be a quarter pitch one for each of the transmission wavelengths in order to obtain the desired focusing. This means that the wavelengths used may not be spaced far from each other. Therefore, such a device cannot be used simultaneously for the two possible windows in telecommunication, i.e. the range of wavelengths between 0.8 and 1.1 .mu.m and the range of wavelengths between 1.3 and 1.6 .mu.m.
If the graded-index cylindrical lens is replaced by a normal lens this will not solve all the problems. In particular, this will not solve the problem of the adjustment of the device by a displacement of the optical fibres. Moreover, it will be very difficult to manufacture such a lens.